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He also revealed, “We had a version of [Before Sunset] that happened two
years later. But once it took us five years and we can’t get the
money to make that movie we have to rethink it. So elements or lines
from that script stayed. When we first finished Before Sunset there
was an idea that we would do. Julie and I wanted to do a short film
the next year that we thought would be really fun to do like a five
minute short and then put it in the vault so to speak and then fifteen
years later make another film.”

The article also discussed the third film:

If you’re a fan (like me) of these two amazing movies and are hoping
they’d do a third film, it sounds like they aren’t against it, but
don’t expect to see it anytime soon. That’s because even though
people constantly give Hawke ideas for the film, he says, “My thought
is we will make one as soon as everybody has forgotten about it.”

Hawke reveals that he, Delpy and Linklater are negotiating a third
outing for the transatlantic couple. "All of three of us have been
having similar feelings that we're ready to revisit those characters,"
said Hawke. "There's nine years between the first two movies and, if
we made the film next summer, it would be nine years again, so we
really started thinking that would be a good thing to do. We're going
to try to write it this year."

So to summarise, they wanted to release Before Sunset earlier, but struggled to get funding and that delayed the launch. When they did finally release the film, nine years had passed. They didn't want to release Before Midnight too early, and eventually settled on releasing it nine years after Before Sunset, to have the same timeline consistency between the first two films as the second and third films.